Precision pointing of an antenna LOS is used to accurately direct transmitting or receiving of an RF beam of an antenna to a receiving or transmitting antenna at a distance from the antenna. For spacecraft RF antennas, high precision is required because even small deviations from a correct LOS can result in a very large drop in signal power from transmitting antennas to receiving antennas. This is due to the fact that there is a large distance between spacecraft or ground antennas that transmit RF signals and ground or spacecraft antennas that receive the signals.
Many spacecraft RF communication and radar antennas require precision LOS pointing to meet mission objectives. Alignment, launch shift, deployment error and thermal deformation can all cause pointing errors. Alignment error is antenna miss-pointing error due to the fact that the LOS of an antenna is not aligned in the required direction. Launch shift error is antenna LOS shift caused by a rocket launch. Deployment error results from inaccuracy of a deployment mechanism. Thermal deformation error is caused by temperature change induced antenna structure deformation when the spacecraft is orbiting in its orbit. Alignment error, launch shift, and deployment error can be measured and corrected during spacecraft initial orbit calibration. Thermal antenna deformation, however, can not be completely calibrated and usually causes a significant amount of spacecraft antenna LOS pointing error during on-orbit operation.
A ground based beacon is often used to measure spacecraft RF antenna LOS pointing errors during operation on-orbit. These errors include the error caused by antenna structural thermal deformation. For commercial spacecraft, maintaining a ground based beacon for about 15-18 years is very expensive as equipment, facilities and human operation can easily cost customers millions of dollars. In many military applications, ground based beacons cannot be used because of their vulnerability to enemy attacks. Sometimes, in commercial applications, ground based beacons are unavailable in the antenna coverage area of the spacecraft.